1. Field of the Invention
The present invention relates to a remote monitoring system, and more particularly to a remote monitoring system having a centralized monitoring station for collecting data from a plurality of monitored stations that are connected to a bus network.
2. Description of the Prior Art
Conventional remote monitoring systems employ a polling process as a data collecting process for requesting all monitored stations to transmit data. In such a polling process, all the monitored stations are polled in the same manner irrespective of the condition of the monitored stations.
The time established by the remote monitoring system to collect data from all the monitored stations is determined by the sum of times required to collect data from the individual monitored stations. If the proportion of data of less importance is large in the amount of data to be transmitted from the monitored stations and if there is even one monitored station which transmits a large amount of data, then the remote monitoring system needs a long period of time to collect the data.
Various data collecting systems have been proposed to solve the above problem. For example, Japanese patent laid-open No. 270426/90 discloses a communication control system in which monitoring information is given preferential ranking, and stored in a plurality of registers according to the given preferential ranking, so that emergency information can be transmitted and received more quickly than the other information.
Japanese patent laid-open No. 75146/92 discloses a centralized monitoring control system in which each monitored station is polled for confirming an amount of data to be transmitted therefrom, and the number of times that each monitored station is to be polled is determined on the basis of the result of polling.
The communication control system disclosed in the former publication is disadvantageous because it fails to handle any monitor information which is given priority ranking in excess of the number of transmission and reception registers that are installed.
The centralized monitoring control system disclosed in the latter publication also has a problem in that it requires an extra period of time to effect polling on all the monitored stations to check the amount of data to be transmitted therefrom.